In a notebook personal computer (hereinafter, referred to as “note PC” for simplicity), which is an example of a portable or mobile type electronic device, power consumption increases as the operating frequency of a CPU increases; and on the other hand, it is requested to provide a longer operation time in a mobile environment and to be smaller in size and lighter in weight. Therefore, most rechargeable batteries mounted on the note PC take a form of a battery pack which includes a plurality of battery cells composed of a lithium ion rechargeable battery having high energy density and in which the battery cells are combined together by series or parallel connection and are accommodated in a housing.
Upon charging/discharging the lithium ion rechargeable battery, it is necessary to precisely control charging/discharging current and voltage. In particular, it is necessary to strictly control the charging voltage in a constant-voltage control period. Therefore, in a battery pack using the lithium ion rechargeable battery, an MPU (microprocessor) is provided within the battery pack. The battery pack generally employs a scheme called a smart battery in which an MPU monitors an internal state of the battery pack during charging and discharging to thereby send information to a note PC body or to activate a protection circuit. The smart battery is a battery device that is compliant with the standards called smart battery system (SBS), initiated by Duracell Inc. (US) and Intel Inc. (US). A battery pack compliant with the above standards is also called an intelligent battery.
In the intelligent battery, an electric circuit portion having, mounted on a substrate, an MPU, a current measurement circuit, a voltage measurement circuit, a remaining capacity calculation circuit, a temperature sensor, and the like, and a plurality of battery cells are accommodated in a housing. The MPU is operable to communicate with an embedded controller of the note PC body via data lines. An overvoltage protection circuit is also installed in the intelligent battery; therefore, when the voltage of the battery cell experiences overvoltage during use, a shutoff element provided in a charger circuit can be operated to stop a charging operation.
According to a technology disclosed in Japanese Laid-open (Kokai) Patent Publication No. 2000-166107, a first protection function portion and a second protection function portion are mounted on a lithium ion battery pack, the first protection function portion operates an FET to stop charging upon detection of a cell voltage of 4.3 V, while the second protection function portion blows a temperature fuse with resistor to stop charging upon detection of a cell voltage of 4.5 V. With such a circuit configuration, even when an abnormality has occurred in the first protection function portion, the second protection functional portion backs up the first protection function portion, guaranteeing that the voltage of the battery cell does not exceed an allowable maximum voltage. According to a technology disclosed in Japanese Laid-open (Kokai) Patent Publication No. 2005-323459, an overdischarge/overcharge detection circuit comprises multiple redundant determination circuits with a plurality of different thresholds for overvoltage detection in a plurality of battery cells.
In the overvoltage protection circuit, in order to guarantee that the cell voltage during charging of the battery cells does not exceed an allowable maximum voltage, it is necessary to forcibly and assuredly stop the charging when the cell voltage exceeds a threshold value. On the other hand, when the battery charger is operating in a normal manner based on the setting voltage, it is necessary to prevent malfunctioning of the overvoltage protection circuit. Since the output voltage of the battery charger has an error with respect to the setting voltage and the cell voltage varies with a change in ambient temperature or drift of the battery charger, it is necessary to provide a fixed margin between the setting value of the battery charger and the threshold value of the overvoltage protection circuit.
In recent years, lithium ion rechargeable batteries are requested to provide a higher safety level. For this reason, related business groups are moving to further tighten the safety standards of the lithium ion rechargeable batteries. Specifically, in the past, the standard allowable maximum voltage of the cell voltage was 4.40 V; however, the standard allowable maximum voltage is now lowered to 4.25 V so the charging system is requested to guarantee that the cell voltage during charging does not exceed the allowable maximum voltage.
In a case where a plurality (three to four) of lithium ion rechargeable batteries are connected in series to form a battery set, the battery charger is operated such that the output voltage applied to both ends of the battery set during a constant voltage control period becomes constant. In order to guarantee that during charging of the battery set, the cell voltage does not exceed the allowable maximum voltage, an overvoltage protection circuit is usually provided for stopping the charging when the cell voltage reaches the threshold value. Moreover, as a backup measure, the cell voltage is controlled so as not to exceed the allowable maximum voltage even when the overvoltage protection circuit is not operating in a normal manner.
According to the conventional overvoltage protection circuit disclosed in Japanese Laid-open (Kokai) Patent Publication No. 2000-166107, in order to guarantee that the cell voltage does not exceed the allowable maximum voltage, the overvoltage protection circuit is duplicated by the first protection function portion that monitors the cell voltage and the second protection function portion. The first protection function portion operates an FET, which is a reversible element, and the second protection function portion operates a temperature fuse, which is a non-reversible element, to thereby stop the charging. Moreover, the threshold values of the first and second protection function portions are set to different values so that the reversible element is operated first. That is, the second protection circuit has the same construction as the first protection circuit so that a prefect backup function can be carried out.
The reason the reversible element is set to operate prior to the non-reversible element is as follows. In a state where the cell voltage approaches a threshold value at which the reversible element is operated, when the cell voltage temporarily exceeds the threshold value due to an abrupt change in ambient temperature or drift of the overvoltage protection system, the charging is preliminarily stopped by the reversible element. However, once the safety is confirmed, the reversible element returns to its original state so that the charging can be resumed. The reason the non-reversible element is set to operate later is as follows. In order to definitely prevent a serious accident which is likely to cause fire hazard, the non-reversible element permanently disables the use of the battery pack when the charging voltage exceeds the threshold value.
Such a construction of the duplicated overvoltage protection circuit composed of the reversible element and the non-reversible element as disclosed in Japanese Laid-open (Kokai) Patent Publication No. 2000-166107 is generally employed in a note PC. In such an overvoltage protection circuit, the charging is not stopped when the charging is performed in a normal manner. It is therefore necessary to set the setting voltage of the battery charger to a low value with a margin relative to the lower threshold value at which the reversible element is operated. However, when the setting voltage of the battery charger is lowered, the full charge capacity decreases or the time to reach the full charge capacity increases, which is therefore undesirable.
According to the determination circuit disclosed in Japanese Laid-open (Kokai) Patent Publication No. 2005-323459, the determination circuit is duplicated or triplicated so that even when one the determination circuits has a fault, the determination is continued by the majority rule. The determination circuits have mutually different overcharge threshold values 3.75 V, 4.0 V, and 4.25 V. Therefore, unless the setting voltage of the battery charger is set to a value lower than the lowest threshold value of the determination circuits, the determination circuit may detect an abnormality as to the overcharging.
In the past, where the allowable maximum voltage of the battery cells is 4.40 V, the setting voltage of the battery charger can be set to 4.20 V for each battery cell even when the overall error of the charging system is considered; therefore, there was no problem concerning the full charge capacity. However, when the allowable maximum voltage is lowered to 4.25 V and when a duplicated overvoltage protection circuit is employed and the setting voltage of the battery charger is lowered by the difference of 0.15 V, it will give rise to another problem that the full charge capacity decreases. It is therefore necessary to provide an overvoltage protection system capable of guaranteeing that the cell voltage does not exceed the allowable maximum voltage in a manner different from the conventional duplicated or triplicated overvoltage protection method.
Accordingly, a compelling need has been recognized in connection with addressing such challenges.